Improved Hydrogen Storage in Magnesium Hydride Catalyzed by

Mar 9, 2009 - and/or amorphous phase on the surface of MgH2 play a crucial role in ... hydride, performed by using a relatively short time ball millin...
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J. Phys. Chem. C 2009, 113, 5324–5328

Improved Hydrogen Storage in Magnesium Hydride Catalyzed by Nanosized Ti0.4Cr0.15Mn0.15V0.3 Alloy X. B. Yu,*,† Y. H. Guo,† H. Yang,*,‡ Z. Wu,‡ D. M. Grant,§ and G. S. Walker*,§ Department of Materials Science, Fudan UniVersity, Shanghai 200433, China, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, Shanghai 200050, China, and School of Mechanical, Materials and Manufacturing Engineering, UniVersity of Nottingham, UniVersity Park, Nottingham NG7 2RD, United Kingdom ReceiVed: NoVember 29, 2008; ReVised Manuscript ReceiVed: January 26, 2009

MgH2 surface modified with nanosized Ti0.4Cr0.15Mn0.15V0.3 alloy was prepared by a short ball mill period with hydrogenated Ti0.4Cr0.15Mn0.15V0.3 alloy. The catalyzed MgH2 showed attractive absorption/desorption properties, desorbing 5.7 wt % hydrogen in 30 min at 290 °C and absorbing more than 90% of its initial hydrogen capacity with 100 min even at below 100 °C. Moreover, the hydrogen capacity did not show any significant decrease after 73 cycles. It was proposed that the BCC particles with a highly dispersed nanosize and/or amorphous phase on the surface of MgH2 play a crucial role in significantly improving the kinetic properties of the MgH2. Introduction Using hydrogen as an energy carrier is an environment friendly approach that produces almost zero pollution emissions from power generators such as fuel cells. However, during the last several decades, the efficient and safe storage and transportation of hydrogen have been major concerns in the use of hydrogen as a fuel. To address these issues and to develop hydrogen-fueled vehicles and portable electronics, research focus has been placed on the development of new materials that can store large amounts of hydrogen at ambient temperature and relatively low pressures.1,2 A design target for automobile fueling has been set by the U.S. Department of Energy at 6.0 wt %.3 Because of the relatively high capacity of 7.6 wt % and low cost, magnesium is considered as a promising candidate for hydrogen storage for automobile applications. However, the absorption/desorption process of hydrogen from magnesium is very slow and requires temperatures above 300 °C.4 Efforts to improve this have involved the modification of both composition and structure of the subject materials to lower the operation temperature so that they lie within a suitable thermodynamic range. A successful example is the formation of ternary hydride: Mg2NiH4,5,6 which has a hydrogen sorption temperature of about 200 °C. However, the theoretical storage capacity of Mg2NiH4 is decreased to 3.6 wt %. Alternatively, the hydrogen sorption properties of magnesium can be greatly improved if the materials are prepared with nanostructures through chemical/physical synthesis7-10 or high-energy ball milling with various additives.11-18 The main contributions to the improvement on hydrogen sorption properties when the magnesium are nanostructured are therefore sought in the increase of the specific surface area, the decrease of diffusion path lengths, and factors concerning the nucleation and growth in nano particles. However, the long ball milling time, which is needed (typically >20 h) for the formation of the nanocrystalline microstructure and a preferable distribu* Corresponding authors. E-mail: [email protected]; huiyang@ mail.sim.ac.cn; [email protected]. † Fudan University. ‡ Chinese Academy of Sciences. § University of Nottingham.

tion of additives on the surface of magnesium, is energy intensive, especially for batch milling large quantities of materials. Recently, great new efforts have been pursued to improve the hydrogen sorption properties based on surface modification with nanoscaled transition metals and oxides.19-21 It was claimed that the doped metals and oxides uniformly distributed on the magnesium surface in the form of nanoparticles, which are believed to facilitate the dissociation/recombination of hydrogen on magnesium surface and therefore catalyze H-sorption in magnesium. Particularly, Hanada et al.22 investigated the catalytic effect of nanoparticles of 3d transition metals on the hydrogen storage properties of MgH2. They found that the 2 mol % Ni nanodoped MgH2 composite prepared by mechanical milling with short time and low rpm exhibited the most significant improvement of the kinetics of the dehydrogenation of MgH2. It was shown that a large amount of hydrogen (6.5 wt %) can be released in the temperature range from 150 to 250 °C at a heating rate of 5 °C min-1 under a He flow with a sufficiently low partial pressure of hydrogen. Here we report the significant improvement of the cycling kinetics of MgH2 by surface modified with a nanosized Ti0.4Cr0.15Mn0.15V0.3 hydride, performed by using a relatively short time ball milling treatment of 1 h. It is known that the Ti-V-based alloys exhibit fast kinetics even at room temperature and can absorb ∼4 wt % of hydrogen at room temperature in a few minutes.23 After hydrogenation, the formation of the hydride phase is accompanied by a volume expansion of up to 18%,24 resulting in a phase transformation from BCC to FCC verified by X-ray diffraction (XRD) as shown in Figure S1a and S1b. Such a phase transformation induces stress and cracking of alloy particles. Furthermore, the ball milling will aid crack propagation resulting in further fragmentation of the hydrogenated Ti0.4Cr0.15Mn0.15V0.3 hydride readily forming nanoparticles. Experimental Section MgH2 (99%, Aldrich) was used without further purification. The as-cast Ti0.4Cr0.15Mn0.15V0.3 alloy sample was prepared by

10.1021/jp810504w CCC: $40.75  2009 American Chemical Society Published on Web 03/09/2009

Improved Hydrogen Storage in Magnesium Hydride

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Figure 1. MS (a) and TGA (b) profiles for H2 evolution from 1 h milled (I) MgH2, (II) MgH2/BCC, and (III) MgH2/HBCC. Heating rate of 20 °C/min.

magnetic levitation melting in an argon atmosphere. A 50 g ingot was turned over and remelted for four times to ensure its homogeneity, and the ingots were then mechanically pulverized into particles below 200 µm in diameter. Part of the alloy powder was hydrogenated at 20 bar H2 and RT for 2 h. The MgH2, MgH2 + 20 wt % hydrogenated Ti0.4Cr0.15Mn0.15V0.3 mixture (named as MgH2/HBCC) and MgH2 + 20 wt % ingot Ti0.4Cr0.15Mn0.15V0.3 mixture (named as MgH2/BCC) were introduced into a stainless steel vial together with stainless steel balls. All handling of the powders were performed in a glovebox under a continuously purified Ar atmosphere (the concentration of both oxygen and water was less than 1 ppm). The sample was ball-milled (EPX 800 Spex shaker ball mill) for 1 h under an inert gas (Ar). Hydrogen release measurements were performed by thermogravimeteric analysis (TGA; TA instruments STD 600) connected to a mass spectrometer (MS; Hiden HPR20) under 1 atm of argon and at a purge rate of 200 cm3 min-1. Typical sample quantity used was ∼20 mg. Hydrogen uptakes and cycling measurements were carried out on an intelligent gravimetric analyzer (IGA, Hiden, U.K.) on a sample of ∼60 mg. Hydrogen gas with a pressure of 20 atm for absorption and 0.1 atm for desorption was introduced into the reaction chamber, the variation of pressure (hydrogen capacity) with time was recorded automatically by the apparatus. PCT measurements (Sievert’s apparatus, AMC, U.S.A.) were performed at 230, 250, and 280 °C in the pressure range 0.01∼3 MPa, the equilibrium time for each point was 30 s. Powder XRD (Bruker D8, Cu KR source) measurements were conducted to confirm the crystalline phase. The surface morphology of the milled mixture was examined by scanning electron microscope (SEM, JSM-6700F) with an energy dispersive X-ray spectrometer (EDS) and transmission electron microscopy (TEM, JEM-1200 EX II). Results and Discussions Figure 1 is the MS/TG profiles for the dehydrogenation of MgH2/HBCC mixture compared to pure MgH2 and MgH2 ball milled with ingot BCC alloy (where HBCC is the hydrogenated BCC alloy). The MgH2/HBCC sample starts to release hydrogen at 255 °C and reaches its peak at 294 °C, which is much lower than that of pure MgH2 and the MgH2/ingot BCC mixture, in which the peak temperatures are 331 and 368-395 °C, respectively. Noting that, after decreasing the heating rate to 2 °C min-1, the MgH2/HBCC sample starts to release hydrogen at 185 °C and reaches its peak at 227 °C (Figure 2). From TGA curves in Figure 1b, the weight loss of MgH2/HBCC mixture is 6.2 wt %, which only shows a slight decrease as compared to that of pure MgH2 (6.9 wt %). Although the peak dehydrogenation temperature for the MgH2/ingot BCC mixture can be

Figure 2. Comparison of MS profiles for H2 evolution from 1 h milled MgH2/HBCC with differing heating rates of 2 °C/min and 20 °C/min

Figure 3. Comparison of MS results for hydrogen evolution for a MgH2/ingot BCC ball milled for 1 (a) and 12 h (b), and a MgH2/HBCC ball milled for 1 h (c). Heating rate was 20 °C/min.

decreased to 308 °C after prolonging the ball milling time to 12 h, it is still 14 °C higher than that of the MgH2/HBCC mixture (Figure 3). It indicates that ball milling with hydrogenated BCC alloy is an effective route to improve the dehydrogenation of MgH2. Figure 4 shows the hydrogenation/dehydrogenation kinetics of the dehydrogenated MgH2 and MgH2/HBCC samples. Before the hydrogenation measurement, the two samples were degassed at 350 °C for 2 h to completely dehydrogenate. In the case of MgH2/HBCC sample, a hydrogen uptake of 5.6 wt % was reached at 93 °C within 100 min. Furthermore, within 500 min, the sample could take-up 4.8 and 3.1 wt % of hydrogen at 55 and 29 °C, respectively. In contrast, the dehydrogenated MgH2 did not take-up any significant amount of hydrogen at 80 °C after 500 min. The dehydrogenation curves in Figure 4b demonstrate that the MgH2/HBCC can release 4 and 5 wt % of hydrogen in 100 min at 200 and 260 °C, respectively. At 290 °C, a hydrogen release of 5.7 wt % was achieved after 30 min. However, no dehydrogenation was observed for ball milled MgH2 under 200 °C. Evidently, the MgH2/HBCC mixture exhibits superior hydrogenation/dehydrogenation properties than the uncatalysed MgH2.

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Figure 4. Hydrogen absorption (a) and desorption (b) curves for 1 h ball milled MgH2/HBCC composite and MgH2 at different temperatures. For hydrogenation measurement, the samples first were evacuated at 350 °C for 2 h to release hydrogen completely. For rehydrogenation measurement, the samples first were hydrogenated fully at 20 atm H2 and 350 °C for 2 h.

Figure 5. Cycling of 1 h milled MgH2/HBCC composite at 290 °C. For each cycle, the sample was hydrogenated at 20 atm hydrogen pressure for 30 min and then dehydrogenated at 0.1 atm hydrogen pressure for 15 min, which resulted in about 75% of dehydrogenation.

Figure 6. XRD patterns for (a) 1 h milled MgH2/HBCC composite and (b) the sample after cycles.

The cycling performance of MgH2/HBCC mixture at 290 °C is shown in Figure 5. The dehydrogenation rate increased with the increasing cycle number in the initial 20 cycles, suggesting an improvement in the kinetics (as each cycle was for a fixed time period). After 20 cycles, a stable dehydrogenation rate of 75% which was maintained for the rest of the experiment, indicating the favorable cycle life of the MgH2/HBCC system. Figure 6 showed the XRD patterns for the 1 h milled MgH2/ HBCC sample before and after being cycled 73 times. After ball milling for 1 h, β-MgH2 phase was observed. The hydrogenated BCC phase cannot be identified, probably due to the peak overlapping by β-MgH2. After cycles, the dehydrogenated MgH2 was changed to sharp Mg peaks, while the BCC presents a broad peak of 2θ centered at ca. 41.5°, suggesting that HBCC was probably in a highly dispersed, nanocrystalline and/or amorphous form after the ball milling and retained this nanostructure even after cycling. The fact that no new compounds formed after cycling the material and that the BCC alloy

particles retained a nanostructure in the magnesium matrix could be one of the main reasons for the MgH2/HBCC to maintain capacity during cycling. This is a big improvement on for example the MgH2/LaNi5 system, in which the hydrogen capacity decreased dramatically after a few cycles due to the formation of Mg2Ni11 and not unsurprising given the relative insolubility of the elements in the BCC alloy with Mg at these temperatures. To explore the possible distribution of BCC hydride on the surface of MgH2, the 1 h ball milled MgH2/HBCC sample was investigated by SEM and TEM techniques. Figure 7a clearly shows that the BCC hydride particles, white particles identified by EDS shown in Figure 7b, P1, are well distributed on the surface of MgH2 with a particle sizes ranging from tens to hundreds nanometers. EDS analysis of the MgH2 matrix (Figure 7b, P2) also identified elements from the BCC alloy, suggesting that some BCC particles were embedded within the MgH2. TEM images in panels c and d in Figure 4 demonstrate that the BCC alloy was crushed into nanoparticles with an average grain size about 30-40 nm, which congregated on the surface of MgH2. The significantly improved hydrogen cycling properties of MgH2/HBCC should be mainly ascribed to the presence of the nanosized BCC particles on the surface layer of the composite sample. These dispersed nanosized BCC particles result in greatly enhanced reduction in hydrogen diffusion path lengths and increased coverage of the MgH2 relative to their conventional coarsegrained counterparts, thus improving the cycling kinetics for MgH2. The activation energy during the hydrogenation/dehydrogenation were estimated by first-order reaction rate after fitting the cycling data.8,25 Figure 8 showed the Arrhenius plots for 1 h ball milled MgH2/HBCC composite. The rate r takes the equation of r ) -ln(1 - Hx/Heq), where Hx is the hydrogen capacity measured by the pressure drop (hydrogenation) or increase (dehydrogenation) in a calibrated volume. Heq is the hydrogen capacity for the complete absorption/desorption of the material. Then, the rate constant of k can be related to Arrhenius equation, k ) A exp(-Ea/RT), where A is a temperature-independent coefficient, Ea is the activation energy of absorption/desorption, R is the gas constant, and T is the absolute temperature. It can be seen that the plot of the rate constant on inverse temperature with a constant driving force displays a linear dependence. The activation energy of hydrogenation and dehydrogenation are 27.74 and 86.34 kJ mol-1, respectively, which is significantly lower than that of the ball-milled MgH2 with a value of 120-142 kJ mol-1.26 The reduction in the activation energy is associated with a change in the rate-limiting step. The desorption of hydrogen from MgH2 is controlled by a slow nucleation and growth process.27 For the MgH2/HBCC

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Figure 7. SEM image of the 1 h milled MgH2/HBCC composite (a) with the EDS analysis (b) and TEM (c and d) with the diffraction pattern (inserted). The EDS revealed that the atomic content of Mg, Ti, V, Cr, and Mn are 9.43%, 37.25%, 24.34%, 8.4%, and 20.58% at P1 and 63.8%, 14.28%, 10.24%, 3.83%, and 20.84% at P2.

Figure 8. Arrhenius plots for 1 h ball milled MgH2/HBCC composite. (a) hydrogenation kinetics and (b) dehydrogenation kinetics.

Figure 9. (a) PCT desorption curves for 1 h ball milled MgH2/HBCC composite at different temperature, (b) van’t Hoff curve [logarithm of the hydrogenation equilibrium hydrogen pressure vs the reciprocal temperature, ln(Peq/P°eq) ) (∆H/R)(1/T) - ∆S/R] for the sample. The equilibrium pressures obtained from the desorption isotherms at 3 wt %. The slope of the line is equal to the enthalpy of the formation divided by the gas constant, and the intercept is equal to the entropy of the formation divided by the gas constant.

composite, the small amount of BCC hydride addition reduces the barrier for nucleation, thus making hydrogen desorption happen at very low driving forces. For the hydrogen absorption, BCC hydride can act as a catalyst for the dissociation of

hydrogen molecules to hydrogen atoms and transfers them to the MgH2/HBCC interface which acts as active nucleation sites for Mg hydride resulting in a decrease of nucleation barrier. On the other hand, the presence of the nanosized BCC particles

5328 J. Phys. Chem. C, Vol. 113, No. 13, 2009 on the surface layer of the composite sample may introduce more defects, which also play an important role in the improvement of kinetics. Figure 9 presents the PCT curves for the 1 h ball milled MgH2/HBCC at various temperatures. No apparent absorption plateau was observed for the sample at 230 °C. However, at 250 and 280 °C, the absorption curves showed a flat plateau at pressures of 0.38 and 0.65 atm. Furthermore, three flat plateaus at pressures of 0.09, 0.22, and 0.45 atm are observed, respectively, from the desorption curves at the above three temperatures. The difference of the curves for absorption and desorption may result from their different thermodynamics and kinetics. From the slope of the van’t Hoff plot from the PCT data, the enthalpy for the MgH2/HBCC composite was calculated to be 73 kJ/mol H2, which is similar to that of bulk MgH2 (74 kJ/mol H2),28 proving the enhanced cycling properties were due to improved kinetics as opposed to a change in the thermodynamics. Conclusions In summary, the surface modification of MgH2 with nanosized BCC was achieved by a short time ball milling. The catalyzed MgH2 exhibited greatly improved hydrogen cycling characteristics as compared to pure MgH2. It was proposed that the BCC particles with a highly dispersed nanosize and/or amorphous phase on the surface of MgH2 play a crucial role in significantly enhancing the kinetics properties of the MgH2. However, the PCT results showed that the hydrogenation plateau pressure for the MgH2/HBCC composite is still low, suggesting that a combined approach, i.e. alloying with magnesium hydride to improve the thermodynamics, leading to an enhanced plateau pressure, is still required for a practical application. Acknowledgment. The authors would like to acknowledge prof. Jun Shen for his help on TEM measurement and discussion. This work was partially supported by the funding of EPSRC-UK, the Hi-Tech Research and Development Program of China (2007AA05Z107 and 2006AA05Z136), the Shanghai Leading Academic Discipline Project (B113), and the Shanghai Pujiang program (08PJ14014). Supporting Information Available: Additional figure as detailed in the text. This material is available free of charge via the Internet at http://pubs.acs.org.

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